The invention relates to a photometric circuit, and more particularly, to a photometric circuit including a single operational amplifier which is adapted to produce a photometric output selectively in response to outputs from a plurality of photoelectric transducer elements which are provided for the purpose of photometry.
As is well recognized, the photometry may utilize a plurality of photoelectric transducer elements having different photometry responses such as those directed to a spotwise photometry and those directed to an average photometry. Alternatively, the photometry may utilize a plurality of photoelectric transducer elements disposed in different photometric optics, such as those used for a TTL (through-the-lens) open photometry and those directed to a TTL film surface reflection photometry. It may be desired to select one of these photoelectric transducer elements for connection with an associated amplifier in order to produce an photometric output in accordance with the response of this particular transducer element. In such instance, there may be provided means for providing a switching among the plurality of photoelectric transducer elements, or means may be provided which switches from one output to another. The transducer element switching means is illustrated in FIG. 1 where it will be noted that a plurality of photoelectric transducer elements SPD1, SPD2 (two in number) are connected to a single amplifier AMP1 through a changeover switch SW1. Depending on the position assumed by the switch SW1, a photometric output may be produced selectively from either the transducer element SPD1 or SPD2. Output switching means is illustrated in FIG. 2 where it will be noted that each of the transducer elements SPD1, SPD2 is connected to an associated amplifier AMP2, AMP3, respectively, while a changeover switch SW2 is connected to the individual outputs of the amplifiers so that a photometric output from either amplifier may be selectively produced depending on the position of the switch SW2.
It will be noted that in the arrangement of FIG. 1, a switching operation takes place on the input side of the amplifier AMP1 having a very high impedance. Accordingly, a high reliability as well as a high contact efficiency is required of the changeover switch SW1, causing an increased cost thereof. In addition, the arrangement is vulnerable to noise applied to the switch, leakage current or poor contact thereof. The arrangement shown in FIG. 2 requires a separate amplifier for each of the transducer elements, and hence the photometric circuit becomes bulky and complicated. In addition, it is difficult to achieve a balance between the responses of the individual amplifiers. If an imbalance is allowed to remain, there results an imbalance in the photometry response.